Multi-wavelength semiconductor lasers

ABSTRACT

A multi-wavelength semiconductor laser is formed by monolithically integrating a plurality of laser diodes ( 1, 2 ) with at least one isolator section ( 3 ) and a coupler ( 4 ), which couples the different emission wavelengths λ 1 , λ 2  into one output port ( 5 ). The isolator section can be either a light absorptive type or wavelength selective type, including a Bragg grating type isolator or a photonic bandgap crystal type isolator. The coupler is preferably a Y-junction coupler, but can also be a multi-branch waveguide coupler or a waveguide directional coupler.

FIELD OF THE INVENTION

The present invention pertains to the field of semiconductor laserfabrication and in particular to a method for manufacturing integratedsingle output multiple wavelength lasers and related photonic integrateddevices to be utilized in wavelength division multiplexing (WDM) opticalcommunication system, optical recording and measurement

BACKGROUND OF THE INVENTION

Multi-wavelength laser sources are key elements for a variety ofapplications such as wavelength division multiplexing (WDM), opticalrecording, color displays and optical color printing. Several methodshave been proposed and patented for the realization of thesemulti-wavelength laser sources and the integration with modulators, ascan be seen in U.S. Pat. No. 4,955,030 (Menigaux), U.S. Pat. No.4,831,629 (Paoli), U.S. Pat. No. 4,993,036 (Ikeda), U.S. Pat. No.5,384,797 (Welch), U.S. Pat. No. 5,519,721 (Takano). They can berealized by the non-uniform current pumping resulting in areas withdifferent band filling, selective area epitaxy or quantum wellintermixing. The last two methods can also be used to fabricate thelasers integrated with modulators.

However, many of the multiple wavelength laser sources or arraysfabricated rely on a large spatial separation between differentwavelength elements, and therefore complex focusing optics are requiredto align the individual lasers to the optical fiber. It is desirablethat all the laser emissions are combined monolithically together on awafer so that the alignment restrictions can be eliminated. One methoduses serially aligned semiconductor lasers each with a directionalcoupler [see U.S. Pat. No. 5,233,187, Japanese Patent 05090715, EuropeanPatent 496348, Sakata]. An inevitable drawback of this method is thatthere cannot be many wavelengths integrated together otherwise thedevice will be too long and the internal loss will be large. Also thegrating must be optimized to act both as a directional coupler and thewavelength selective reflector. Another method is to combine parallelaligned laser arrays into one output port by use of appropriatewaveguide alignment, such as the Y-junction coupler, rib waveguidecoupler structures etc [See Japanese Patent 24209, 58175884, IEEEPhoton. Technol. Lett. Vol. 7, pp. 944-946, 1995]. One problem arisingfrom the combination of different wavelengths together is that the crosstalks among these wavelengths are inevitable without additionaltreatment. The longer wavelength laser element will be optically pumpedto lasing even only when the shorter wavelength laser element iselectrically biased. The consequence is that each wavelength is notindependently addressable. One method is to add an electroabsorptiontype modulator to each laser element. This method is a littlecomplicated since it needs a lot of additional electrodes to controleach modulator.

In this patent we propose a new method, the introduction of an isolator,to provide the needed isolation among different wavelength laserelements. It does not need additional electrodes so it is simple tofabricate and easy to operate. We also provide the method of fabricatingthe said isolator and other integrated optoelectronic devices.

SUMMARY OF THE INVENTION

The invention has as its objective, to introduce an isolator orisolators monolithically integrated with the semiconductor laser arraysand to provide an easy method of producing integrated multiplewavelength semiconductor lasers that can independently emit eachwavelength in one spot.

According to one aspect of the invention, there is provided Amulti-wavelength semiconductor laser comprising:

-   -   a plurality of laser diode sections in parallel arrangement and        having different emission wavelengths relative to each other;    -   at least one isolator section, the or each of said isolator        sections being integrated with a respective one of said laser        diode sections;    -   a coupler integrated with the laser diode sections and the at        least one isolator section to couple the different emission        wavelengths into one output port.

The multi-wavelength semiconductor laser comprises three essentialcomponents: laser diode sections; at least one isolator section; and acoupler. The cavities of the laser diode sections form a parallel arrayto provide different emission wavelengths with each wavelengthindependently addressable. The coupler is used to couple the differentwavelengths into one output port. The coupler may be, for example, aY-junction coupler, a multi-branch waveguide coupler or a waveguidedirectional coupler such as a rib waveguide directional coupler. Theisolator section is used to isolate the cross talks among the differentwavelengths so as to ensure that the device can emit either eachwavelength singly or their desired combinations.

The isolator section may be, for example, either a wavelength selectivetype or a light absorptive type. For the first type, it can be adistributed Bragg grating type isolator or a photonic bandgap crystaltype isolator which is transmissive to one wavelength of the laser diodesection but highly reflective to other wavelengths. For a dualwavelength laser source comprising two laser diode sections, an isolatormay be integrated with one or each of laser diode sections and tuned toreflect the other wavelength which is not desired. For the second typeof isolation, the isolator has a band gap energy transparent to thelaser wavelength of the laser diode section but absorbing to all otherlaser wavelengths. There is no need for an electrical contact for theisolation region thus giving rise to ease of fabrication and operation.

In another embodiment, the multi-wavelength semiconductor lasercomprises four laser diode sections, at least three of which areintegrated with a respective isolator section. In this case, the couplermay include a series of Y-junction couplers coupled by intermediateelements or may include a four-branch coupler.

The integrated multi-wavelength laser source is advantageouslyfabricated using a quantum well based heterojunction semiconductor laserstructure. The semiconductor substrate is preferably made of either GaAsor InP. The quantum well can be made of GaAs, InGaAs, InAlGaAs, orInGaAsP. The device can be made either by selective area metal organicvapor phase epitaxy (MOVPE) or by post growth quantum well intermixingtechnology. By controlling the area of the semiconductor laser surfacecovered by the intermixing inhibiting layer, such as the Ge layer, a onestep selective area impurity free quantum well intermixing can be usedto fabricate such devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic waveguide structure of a dual wavelength lasersource with monolithically integrated Y-junction coupler and isolator.FIG. 1 b is a diagram of the device with ridge waveguide and verticallayer structure shown.

FIGS. 2 a and 2 b illustrate the band diagram and the experimentalresults of the relative photoluminescence spectra for the four differentsections in the dual wavelength laser source after a one step selectivearea intermixing.

FIGS. 3 a and 3 b are schematic waveguide drawings for two types offour-wavelength laser sources with monolithically integrated Y-junctioncouplers and isolators.

FIG. 4 is a schematic waveguide drawing for a dual wavelength lasersource with monolithically integrated Y-junction coupler and distributedBragg gratings.

FIGS. 5 a and 5 b are schematic diagrams of a Y-junction coupler and arib waveguide directional coupler.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 a, there is shown a schematic waveguide structure ofa dual wavelength laser source monolithically integrated with anisolator and a Y-junction coupler. It contains laser diode sections 1and 2, an isolator 3 and a Y-junction coupler 4. The structure includescleaved laser facets 5.

The Y-junction coupler 4 is used to direct the two signals from thelaser diode sections, (i.e. gain regions) 1 and 2 into a single outputport. The length of the gain region and isolation region are 700 μm and300 μm respectively. The Y-junction coupler 4 is composed of two S-bendswith a radius of curvature of 1328 μm. The central space between the twobranches is 34 μm. The bending loss in the Y-junction waveguide shouldbe kept small in the design. FIG. 1 b shows the schematic diagram of aridge waveguide and vertical layer structure. The ridge waveguide can beformed by either wet chemical etching or Plasma dry etching (ICP orRIB). The wafer is a graded index separate confinement heterojunction(GRINSCH) laser structure. The n-type substrate 7 can be either GaAs orIhP or other relevant semiconductors. A lower cladding layer 8 isprovided with n-type doping to 2×10¹⁸ cm⁻³ while an upper cladding layer12 is provided with p-type doping to 5×10¹⁸ cm⁻³. There is also providedlower and upper confinement layers 9 and 11, wherein 10 is the activeregion with a quantum well structure, and metal contacts 14 and 15 forthe electrode.

The gain regions 1 and 2 have different wavelengths λ₁ and λ₂. TheY-junction coupler region has a bandgap with an equivalent wavelength λ₄that must be short enough to ensure that it is completely transparent tothe laser wavelengths λ₁, λ₂. The isolator 3 has an equivalentwavelength λ₃ that should be low enough to be transparent to λ₁ but beabsorbing to λ₂ so that the laser light from channel 2 will notoptically pump channel 1 when only channel 2 is electrically biased. Asa result, the two lasers can be operated either simultaneously orseparately. Different band gap regions in the wafer can be realized byeither selective area MOVPE or by a one step selective area-quantum wellintermixing.

FIG. 2 a shows a schematic band diagram illustrating the four differentbandgap regions realized by quantum well intermixing. The energy levelsin the quantum well are represented as numerals 22, 23, 24 and 25 forlaser diode section 1, isolator section 3, laser diode section 2, andY-junction coupler section 4 respectively. Due to the interdiffusion ofthe well and barrier elements, the quantum well shape and compositionchanged thus leading to blueshift of the quantum well energy levels. Thep and n type cladding layers are represented as 16 and 21 respectively.

Graded index layers 17 and 20 and separate confinement layers 18 and 19are provided. Taking the InGaAs/GaAs laser sample with a single 8 nmwide In_(0.2)Ga_(0.8)As quantum well confined by two 10 nm GaAs barriersas an example, FIG. 2 b shows the low temperature photoluminescencespectra of the sample after using the technology of quantum wellintermixing with a Ge interlayer controlling band gap tuning. Bydefining the area of the wafer covered with a Ge layer by varying thespacing between the 1 μm wide Ge strip array, different intermixingdegree and hence, different emission wavelengths can be achieved. The Gecoverage for the four regions, 1, 2, 3, 4, in this device are 100%, 45%,25% and 0% respectively. The Ge strip arrays were formed by evaporatingGe onto the photo-resist patterned GaAs surface followed by lift-off.The whole sample surface was then covered by SiO₂ and undergo rapidthermal annealing. The as-grown sample has a peak wavelength at 916 nmor 1.353 meV. The peak position of the PL signals for Y-junction coupler4 (0% Ge cover) are 77.8 meV and 52.4 meV larger than that of 1 (100% Gecover) and 2 (45% Ge cover) respectively. The bandgap of isolator (25%Ge cover) is 43 meV larger than channel 1 and 18 meV larger than channel2. It is transparent to channel 1 but partly absorbing to channel 2.

FIG. 3 a and FIG. 3 b show schematic waveguide diagrams of afour-wavelength laser source emitting from one output port. Fourdifferent laser diode sections 26 ₁, 26 ₂, 26 ₃, and 26 ₄ withwavelengths of λ₁, λ₂, λ₃, λ₄, respectively are provided as are isolatorsections 27 ₁, 27 ₂, 27 ₃ and 27 ₄. It is noted that the isolator thatis in the path of the shortest wavelength laser diode section is notnecessary. If we assume λ₁>λ₂>λ₃>λ₄, then isolator 27 ₄ can be saved andreplaced with the material of the same band gap as the Y-coupler.

In FIG. 3 a, the four channels 26 are coupled by two Y couplers 28 to aset of two identical intermediate elements 29, which in turn are coupledby Y coupler 30 to the output waveguide 31. In FIG. 3 b, the fourchannels are coupled by one coupler 33 to the output waveguide 35. Somespecial treatment should advantageously be taken to this four-branch “Y”coupler, such as the employment of flank wings at the outmost twobranches to ensure equal power division in the four branches.

As an example, we assume the energy level of the InGaAs/GaAs Quantumwell structure sample, that is for 26 ₁ or λ₁, is 1.360 eV. Then theenergy levels for the other three branches of the laser diode sections26 ₂ (λ₂), 26 ₃ (λ₃), and 26 ₄ (λ₄), with equal spaced wavelengthdifference of 12 meV, could be 1.372 eV, 1.384 eV, and 1.396 eVrespectively. The energy level for the isolators 27 ₁, 27 ₂, and 27 ₃could be 1.39 eV, 1.402 eV and 1.414 eV. The band gap of the Y couplerregion could be 1.440 eV. So the isolator 27 ₁(27 ₂, 27 ₃) istransparent to laser diode 26 ₁ (26 ₂, 26 ₃) but is absorptive to theother three laser diodes 26 ₂, 26 ₃, and 26 ₄ (26 ₁, 26 ₃, 26 ₄; 26 ₁,26 ₂, 26 ₄). The wavelength spacings between the two adjacent gainregions can be changed within a range of 10 to 17 meV. These bandgaptuning can be realized by selective area quantum well intermixingcarried out on the same wafer, as the method described above, or byselective area MOVPE.

Another type of isolators 38, 39 are shown in FIG. 4. They are in theform of distributed Bragg gratings, photonic bandgap crystals, or astack of layers that are designed to be transmissive only to the laserwavelength of their respective gain regions 36 and 37. Here thedistributed Bragg grating does not act as a reflector but as awavelength filter. The two cleaved facets are still the two mirrors toconstruct the laser cavity. If the laser diode section 36, 37 aredistributed feedback (DFB) laser or distributed Bragg reflecting (DBR)laser, then there will be two different pitch gratings along the lightpropagation direction. For a dual wavelength integrated laser source,isolators 38 and 39 can be a conventional distributed Bragg grating butbe highly reflective to the laser wavelength of their opposite gainregions 37 and 36, rather than 36 and 37, respectively. Using thegrating as the isolator, the wavelength difference between the adjacentgain regions can be made very small because of the very sharp wavelengthselection characteristics of the gratings. Such an arrangement has abetter performance in isolating the crosstalks among different laserwavelengths than the absorption type isolator. However it is morecomplicated in design and fabrication.

The Y-junction coupler 40 is a 3 port device arranged so that opticalpower fed into one port is divided equally between the other two ports.One physical form that such a coupler can take is that of a single modewaveguide 41 (FIG. 5 a) which divides at a small angle into two singlemode waveguides 42, 43. An alternative form is provided by a balanceddirectional coupler configuration in which the first waveguide 44 (FIG.5 b) terminates in a region where it is symmetrically flanked on bothsides by the start of two other waveguides 46,47. The overlapping of thewaveguides is such that power launched into the Y-junction coupler viawaveguide 44 is fully coupled equally into the other two waveguides sothat none is reflected at the end 45 of the first waveguide. The lateralseparation of the two ports should be sufficient to provide no mutualevanescence coupling.

In the above explained embodiments, the practice of the invention hasbeen demonstrated in the examples using GaAs type semiconductors.However, it can be effectively applied to all III-V type semiconductors.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withoutdeparting from the true scope and spirit of the invention in its broaderaspects.

1. A multi-wavelength semiconductor laser comprising: a semiconductorsubstrate; first and second laser diode sections, formed on saidsemiconductor substrate in parallel arrangement, and having differentemission wavelengths relative to each other; an isolator section formedon said semiconductor substrate, and being integrated with said firstlaser diode section for passing emissions by said first laser diode andisolating said first laser diode section from emissions of said secondlaser diode section; a coupler integrated on said semiconductorsubstrate with said first and second laser diode sections and saidisolator section to couple said different emission wavelengths into oneoutput port.
 2. A multi-wavelength semiconductor laser according toclaim 1, wherein said isolator section is a light absorptive typeisolator.
 3. A multi-wavelength semiconductor laser according to claim1, wherein said isolator section is a wavelength selective typeisolator.
 4. A multi-wavelength semiconductor laser according to claim3, wherein said isolator section is a Bragg grating type isolator orphotonic bandgap crystal type isolator which is transmissive to awavelength which is equivalent to said emission wavelength of said firstlaser diode section.
 5. A multi-wavelength semiconductor laser accordingto claim 1, wherein said coupler is a Y-junction coupler, a multi-branchwaveguide coupler or a waveguide directional coupler.
 6. Amulti-wavelength semiconductor laser according to claim 1, wherein saidmulti-wavelength semiconductor laser is a dual wavelength semiconductorlaser.
 7. A multi-wavelength semiconductor laser according to claim 6,wherein said coupler is a Y-junction coupler.
 8. A multi-wavelengthsemiconductor laser according to claim 1, wherein said multi-wavelengthsemiconductor laser comprises four laser diode sections, at least threeof said laser diode sections being integrated with a respective isolatorsection.
 9. A multi-wavelength semiconductor laser according to claim 8,wherein said coupler includes two Y-junction couplers, each of which isintegrated with two of said laser diode sections, a pair of identicalintermediate elements each coupled to a respective one of saidY-junction couplers and a further Y-junction coupler coupled to saidintermediate elements to provide said outlet port.
 10. Amulti-wavelength semiconductor laser according to claim 8, wherein saidcoupler is a four-branch coupler.
 11. A multi-wavelength semiconductorlaser according to claim 1, comprising a quantum well laser structure.12. A multi-wavelength semiconductor laser according to claim 11,wherein said semiconductor substrate is formed from GaAs and saidquantum well is formed from GaAs, InGaAs or InAlGaAs.
 13. Amulti-wavelength semiconductor laser according to claim 11, wherein saidsemiconductor substrate is formed from InP and said quantum well isformed from InGaAsP, InGaAs, or InAlGaAs.
 14. A multi-wavelengthsemiconductor laser according to claim 1, wherein said semiconductorlaser is formed by a process of one of selective area metal organicvapor phase epitaxy (MOVPE) and selective area quantum well intermixing.15. A multi-wavelength semiconductor laser comprising: first and secondlaser diode sections in parallel arrangement formed on a semiconductorsubstrate, said first laser diode section emitting light at a firstemission wavelength, said second laser diode section emitting light at asecond emission wavelength; a coupler comprising fist and second inputsand an output for coupling emissions of said first and second laserdiode sections provided at said first and second inputs respectively tosaid output; and an isolator that passes emissions at said firstwavelength and substantially attenuates emissions at said secondwavelength, formed on said semiconductor substrate between said firstinput and said first laser diode section to isolate said first laserdiode section from emissions of said second laser diode section.